The coupling of alkali metal-terminated polymers such as lithium-terminated polymers is a process known in the art. In accordance with this known process, an alkali metal-terminated polymer is treated with a compound having two or more functional groups containing two or more reactive sites capable of reacting with the carbon-alkali metal bonds of this alkali metal-terminated polymer. The multifunctional coupling agent thereby becomes a nucleus for the resulting structure. From this nucleus long-chain polymer branches radiate and such coupled polymers have specific properties that render them useful for particular applications.
Coupled polymers that are derived from coupling agents possessing three or more reactive sites are frequently called radial polymers. Such radial polymers have been of particular interest because of their increased Mooney viscosity, improved processability, and reduced cold flow as compared to the uncoupled or parent polymers. Hydrogenation of the alkadiene-based radial polymers has received considerable attention in recent years because the hydrogenation of such polymers increases the resistance of such polymers to environmental attack, e.g., oxygen and/or ozone deterioration.
It has, however, been found that some of these radial polymers are not sufficiently stable under hydrogenation conditions. More specifically, it has been found that radial polymers that have a connection between an inorganic atom and the first carbon atom of the parent polymer chain sometimes are not as stable as radial polymers in which the connection between the coupling agent and the polymer is a carbon-carbon bond. If this cleavage of the bond between the inorganic atom of the coupling agent and the carbon atom of the polymer chain occurs, the hydrogenated radial polymer is admixed with hydrogenated but no longer coupled polymer chains. This effect can impair the properties of the coupled polymer. Therefore, it would be desirable to have a coupling agent available which does not contain inorganic atoms connected to the active sites of the coupling agent and at the same time achieves a high degree of coupling efficiency.